The invention relates to a method of applying a thermally black layer to a heating member for an indirectly heated cathode, in which a layer of a suspension of tungsten particles and aluminium oxide particles is provided on the heating member, after which this layer is sintered thereby forming the thermally black layer.
Indirectly heated cathodes are used in, amongst others, cathode ray tubes such as display tubes. Such an indirectly heated cathode comprises a cylindrical member having an electron emitting outer surface and containing in its interior a heating or incandescent member. In general, the heating member is a spiral-shaped tungsten wire which is coated with an electrically insulating aluminium oxide layer. This layer is coated with a black layer to enlarge the heat-radiation from the wire to the cathode, thereby decreasing the warm-up time of the cathode. To this end, the wire is immersed in a suspension of tungsten particles and aluminum oxide particles and after it has been removed it is dried and sintered at a temperature of 1650.degree. C. The black layer obtained is a satisfactory heat radiator.
Such a method is known from U.S. Pat. No. 3,770,601. A disadvantage of the known method is that the suspension used is unstable even if it is stirred. The heavy tungsten particles (specific gravity=19 g/cm.sup.3) coagulate and settle. As a result of this the suspension becomes rich in aluminium oxide particles, such that the tungsten/aluminium oxide ratio in the black layer is no longer correct. Moreover, the pipes of the commonly used circulation system of the coating device become clogged.